Emissions

Emissions are unwanted pollutant species that are by-products of reactions. There are two options to model pollutant species in Simcenter STAR-CCM+. When using the Complex Chemistry model, you can include the pollutant species in the chemical mechanism. For all other models, in particular Flamelet models and the Eddy Break-Up (EBU) model, separate transport equations for the pollutant species are solved.

When modeling NOx emissions using a Flamelet model for which you import a chemical mechanism that contains NOx species, these NOx species should not be selected as species for post-processing within the flamelet table. NOx species should not be stored directly in flamelet tables since NOx species form slowly and are therefore not compatible with the flamelet approximation. A dedicated emissions option is provided within the parameters of the flamelet table generator, which ensures that NOx emissions are transported and calculated accurately.

Simcenter STAR-CCM+ has the following pollutant models:
  • NOx Emission
    • NOx Fuel
    • NOx Thermal
    • NOx Prompt
  • Soot Emissions
    • Soot Moments
    • Soot Two-Equation
    • Soot Sections

NOx Emission Model

Although oxides of nitrogen (NOx) are created naturally from atmospheric nitrogen during thunderstorms, transportation and industries that burn fossil fuels create more significant NOx emissions. NOx emissions contribute significantly to the creation of acid rain. It is therefore important to develop technologies which produce lower levels of NOx emissions wherever possible.

When using any of the reacting flow models, you can activate the optional NOx Emission model. This model provides a tool to predict the NOx production from various sources and helps in the design of NOx control measures.

The NOx Emission model solves the transport equation for the concentration of NOx, using the Fuel NOx model and/or the NOx Thermal model to contribute to the NOx reactive source term. You can also additionally use the NOx Prompt model.
  • The NOx Fuel model contributes the NOx emission arising from fuels such as heavy liquids and coal which release fuel-bound nitrogen during combustion. Therefore, when modeling emissions that are resultant from burning these fuels, use the Fuel NOx model in addition to the Thermal NOx model. In Lagrangian phases, you can use the Fuel NOx model for liquid fuel or coal. In addition to liquid fuels and coal, Simcenter STAR-CCM+ allows you to model Fuel NOx for a purely gaseous fuel. For pure gas fuel, you can introduce the intermediate species, HCN and/or NH3, arising from the fuel directly at the inlet boundary.
  • The NOx Thermal model contributes the NOx emission that is formed from N2 and O2 or OH under lean fuel conditions and at high temperatures, using the three-step extended Zeldovich reaction mechanism. Thermal NOx is dominant in hydrocarbon flames. For combustion set-ups that do not use the Complex Chemistry model, the concentrations of species are computed during the flamelet table generation and are taken directly from the flamelet table for each iteration. When using the Complex Chemistry model, the source terms are computed at each iteration.
  • The NOx Prompt model contributes the NOx emission arising from processes that are not accounted for by the NOx Fuel or NOx Thermal models. For example, atmospheric nitrogen reacting with intermediate species of combustion such as C, CH, and CH2 radicals. Such contributions are generated during low-temperature combustion.

NOx is considered to be a passive scalar, not influencing density calculations. Therefore, in steady-state simulations, use the NOx Emission model as a post-processing tool, that is, activate it after the flow field has converged. In transient simulations, to model the evolution of NOx accurately, activate this model from the beginning.

Soot Emissions Model

The formation and emission of carbonaceous particles is often observed during the combustion of hydrocarbons. These particulates, called soot, are identified in flames and fires as yellow luminescence. In gas turbines, internal combustion engines and other practical combustion devices, the formation of soot is mostly a product of incomplete combustion.

Apart from the resulting loss in combustion efficiency, a serious effect of soot formation is the health hazard. On the other hand, there are situations where the presence or generation of soot is required. For example, generation of carbon black is needed in the production of automobile tires. In furnaces for industrial application or in heat generators, the intermediate formation of soot is required to augment the heat transfer by radiation. The soot, however, must undergo oxidation before the exhaust is released into the environment.

It is widely accepted that the formation of soot is a complex process which consists of the following:

  • Fuel pyrolysis and oxidation reactions
  • Formation of polycyclic aromatic hydrocarbons (PAH)
  • Inception of first particles
  • Growth of soot particles due to reaction with gas-phase species
  • Coagulation of particles
  • Oxidation of soot particles and intermediates

Soot Moments Model

The method of moments is based on the fact that solving an infinite set of equations for the statistical moments of the PSDF is equivalent to the direct simulation of the PSDF. Additionally, it can be shown that often the accuracy using only a few moments is sufficiently high. Usually, a set of equations for the first few moments is applied, where the accuracy of the approach increases with the number of moments that are used. The main advantages are:

  • Computational efficiency
  • The ability to extract major features of the PSDF from the moments, such as:
    • Soot volume fraction
    • Mean number density
    • Total surface
    • Mean diameter

    The Soot Moments model in Simcenter STAR-CCM+ can solve up to four moments: Moment 0, 1, 2, and 3. The model is compatible with the Complex Chemistry, Steady Laminar Flamelet (SLF), and Flamelet Generated Manifold (FGM) combustion models.

Soot Two-Equation Model

Simcenter STAR-CCM+ provides a Soot Two-Equation model that is based on a technique that is called the Moss-Brookes-Hall (MBH) soot model.

The Soot Two-Equation model is a semi-empirical soot model that is based on four physical processes of soot formation:

  • Nucleation / Inception
  • Coagulation
  • Surface growth
  • Oxidation

The Soot Two-Equation model allows you to implement any two-equation soot model by setting the built-in source terms to zero, and then specifying new source terms for each of the four physical processes of soot formation.

When you activate the Soot Two-Equation model with the Eddy Break-up (EBU) model, the fluid stream manager becomes available. The fluid stream manager lets you define the fuel and oxidizer streams.

In Simcenter STAR-CCM+ the Soot Two-Equation model offers a choice of nucleation options in its properties: Acetylene-based and PAH-based.

To access the properties of the Soot Two-Equation model, select the Soot Two-Equation node.

Soot Sections

The Sectional method is based on a description of sections containing soot particles of equal volume, allowing a volume-based discretization of particle sizes together with conservation of the soot number density and mass. A number of sections are transported in the section model.